The present invention relates to an image forming device equipped with a fixing device for heat-fixing toner, such as analog or digital copying machines and facsimile machines, laser beam printers and the like utilizing an electrophotograph method.
In the prior art, the image forming device utilizing an electrophotograph method included forming a visible image by powder toner from a latent image formed on a photosensitive body, and transferring said visible image or toner image to a predetermined sheet of paper, and then melting the toner by heat and pressurizing the same, in order to fix the toner image onto the paper, thereby forming a permanently visible image.
The conventional control block for the fixing device comprises a fixing heater, a temperature detector, a fixing control circuit, a control circuit, a zero-cross circuit and the like.
An AC (alternating current) driven halogen lamp and the like are widely used for the fixing heater conventionally, and a thermistor is generally used as the temperature detector.
The surface temperature of the fixing heater will rise when power is supplied to said heater, and the temperature detector mounted either in the vicinity of or in contact with the fixing heater detects the surface temperature of the heater, and the temperature detection data of the heater is outputted from the fixing control circuit to the control circuit.
For example, a microcomputer is provided In the control circuit. Said microcomputer comprises an analog port, and when an analog voltage Is inputted from the thermistor to the analog port, the analog voltage value is converted to digital data, which will be taken Into the microcomputer in the control circuit.
The analog port has an analog voltage resolving power of approximately 256 level (equally divided into 255 parts) of the reference voltage in general.
Further, the fixing temperature Is set to a temperature enabling heat-fixing the toner on the paper, considering the feeding rate of the paper passing through the fixing device of the image forming device, or the toner material and the like. If the temperature of the fixing heater detected by the temperature detector is lower than said fixing temperature, the signal for providing electricity to the fixing heater will be outputted from the control circuit to the zero-cross circuit, and driving signal is outputted from said zero-cross circuit to the fixing heater, thereby providing electricity to the heater.
On the other hand, when the temperature of the heater being detected by the temperature detection circuit is higher than said fixing temperature, the signal for not providing power to the fixing heater will be outputted from the control circuit to the zero-cross circuit, and the fixing heater is turned off. Thereby, the fixing heater is controlled to maintain a predetermined fixing temperature.
Moreover, since the above temperature control method is controlled by the microcomputer in the control circuit, if the microcomputer becomes out of control by electric noise, disorder, temperature rise and the like, no fixing temperature control will be performed, or the heater lamp will be lighted continuously, generating smoke or fire.
Therefore, the prior art device is quipped with a high-temperature protect circuit comprising only hardware, other than the above-mentioned software process. The detailed explanation of said high-temperature protect circuit is omitted, since it is not directly related to the present invention.
Further, the structure of the conventional fixing device is explained in the following description. The fixing device comprises of a heater lamp, a thermistor, an upper heat roll (fixing roll), and a lower heat roll (pressurization roll).
The heater lamp is mounted in the interior of the rotatable upper heat roll, which is for heating the upper heat roll to a predetermined temperature as was explained above. A silicon rubber and the like are used as the material of the lower heat roll, and a metal pipe treated with TEFLON and the like is used for the upper heat roll.
The thermistor is mounted either in the vicinity of or in contact with the upper portion of the upper heat roll that detects either the surface or approximate temperature of the upper heat roll. According to the detection results, the heater lamp is either turned on or off, in order to control the upper heater roll to maintain an appropriate temperature.
On the other hand, as was disclosed above, the fixing device is positioned in front of a discharge opening in a conveyance passage passing through the image forming portion. There, the paper to where the toner image was transferred is conveyed through the upper heat roll and the lower heat roll, where the toner on the paper is melted by the heat of the upper heat roll, and pressurized between the upper heat roll and the lower heat roll. Thereby, the toner image on the sheet is turned into a permanently visible image.
A halogen lamp is widely used as the heating means for heating the fixing roll.
The outline of the circuit block, the structure, and the temperature control method of the fixing device in the prior art was as explained above. Further, the control method of the fixing temperature is disclosed in Japanese Laid-Open Patent Publication No. 55-89879, Japanese Laid-Open Utility Model Publication No. 1-171471, and Japanese Laid-Open Patent Publication No. 58-54365.
Japanese Laid-Open Patent Publication No. 55-89879 discloses an art for varying the power supply to the heater per period time by determining the temperature varying rate of the heater in the fixing device.
Instead of controlling the fixing temperature to a fixed value, the above publication discloses to vary the temperature control corresponding to the temperature change or heat taken by the paper, and to vary the power corresponding to the temperature varying rate at the initial power turn-on time, thereby reducing overshooting and the like.
Japanese Laid-Open Utility Model Publication No. 1-171471 discloses an art for preventing the deterioration of the fixing roll, wherein during the stand-by state of the image forming device, the fixing roll is driven when the fixing heater temperature is high, and the roll is not driven when the temperature is low.
Japanese Laid-Open Patent Publication No. 58-54365 discloses an art for preventing the temporary stop of the copying operation caused by the reduction of temperature, by temporarily stopping the copying operation when the fixing temperature is reduced during the operation, and reducing the speed of the number of sheets copied per unit time (number of sheets/minute)when continuous printing is restarted.
According to Japanese Laid-Open Patent Publication No. 55-89879, the fixing temperature is not controlled to a fixed temperature, but rather, the temperature varying rate of the heater in the fixing device is determined, and the power supply to the heater per unit time is changed. Further, the temperature control is changed based on the temperature change or heat taken by the sheet paper, and the power is varied similarly by the temperature varying rate at the initial power turn-on time.
Accordingly, the above method is effective in that an appropriate fixing temperature control and reduction of overshooting and the like is possible in a high-speed facsimile device and the like to which a band compression is applied, where the feeding rate of the record paper is not fixed.
Moreover, it is effective in that the initial overshooting and the like could be prevented in the copying machine or the printer whose feeding rate of the record paper is generally constant, when turning the power on after the power has been turned off for a long period of time.
However, according to the disclosure of Japanese Laid-Open Patent Publication No. 55-89879, the method is for controlling the supply of power based on the temperature varying rate among certain reference temperatures being measured. Therefore, in the continuous copying operation by copying machines and printers where the feeding rate (conveyance speed) of the paper is generally constant, which is in a comparatively stable region, the temperature difference between the reference temperature and the measured temperature is comparatively small, and the temperature varying rate is comparatively slow, there was a problem that accurate temperature measurement and temperature control was difficult.
Further, Japanese Laid-Open Utility Model Publication No. 1-171471 discloses driving the fixing roll when the fixing heater temperature is high, and not driving the fixing roll when the temperature is low, during the stand-by state. This is effective in that the deterioration of silicon rubber caused by adding pressure for a long period of time to the fixing roll could be prevented.
However, according to the contents of the Publication, the fixing roll and the heater are only controlled by a predetermined fixing temperature, and the fixing roll is driven for each time of the fixing temperature control. This caused problems such as unnecessary power consumption, and occurrence of noise.
Further, Japanese Laid-Open Patent Publication No. 58-54365 discloses stopping the copying operation when the fixing temperature is decreased while copying, and when continuous copying is restarted, the copying paper speed per unit time (number of sheets/minute) is decreased. This is effective in that copying operation could be continued without stopping the copying operation temporarily when the temperature is reduced.
However, according to the disclosure of the above Publication, instead of the necessity to temporarily stop the copying operation, the copying paper speed is decreased when the temperature is decreased. Therefore, it had problems such as the productivity being dropped compared to normal operation, and the time needed for the copying operation being longer. Accordingly, none of the above-mentioned prior art techniques realized a control method for minimizing the temperature ripple of the fixing device, and preventing reduction of productivity, even when the paper size or commercial power is changed, in the comparatively stable region such as during the continuous copying by the copying machine or printer having a generally fixed conveyance speed of the paper.
On the other hand, a zero-cross control and a phase control were generally known conventionally as the power control method of the heater applied in the temperature control of the fixing process.
The zero-cross control method is a method where the heater is turned on or off at the zero-cross point (zero point) of the AC waveform. This is not suitable for the temperature control of a thin heat roll since detailed power control is troublesome and temperature ripple is comparatively large, but it is advantageous in that occurrence of electric noise is comparatively small.
The phase control method is a control method where power is turned on only between a predetermined phase in the AC waveform. This is suitable for the temperature control of a thin heat roll since detailed power control is possible and temperature ripple is comparatively small, but the occurrence of electronic noise is larger compared to the zero-cross control method.
When the temperature ripple of the heat roll is large, a so-called hot offset may occur where the stain of toner adheres to the heat roll, or fixing disorder due to low temperature may happen.
The restriction against the electronic noise (noise terminal voltage) flowing from the electronic devices to the commercial power source is getting strict year after year. Further, from the energy-saving point of view, in the attempt to shorten the time needed to achieve the printable (image forming) status from the power switch-on time, which is so-called a warm-up time, the thickness of the heat roll in the fixing device is getting thinner year after year.
However, though there is a need to raise the power of the heater in order to shorten the warm-up time, said electronic noise will also be increased when the heater power is increased. Another problem was that temperature ripple was also likely to increase when the heat roll thickness was decreased.